A great many performance attributes are demanded of tires. In particular, there is a demand for steering stability during high-speed traveling, stability on wet road surfaces, reduction in rolling resistance to enable greater automobile fuel efficiency, and improvement in friction resistance.
Conventionally, in order to achieve both a reduction in rolling resistance as well as stability on wet road surfaces, in particular, silica is widely used as a reinforcing filler.
However, since silica has silanol groups on the particle surface, the particles readily agglomerate together, and when used as a reinforcing filler, there is the problem that it adversely affects tire wear resistance and mechanical characteristics.
To combat this, a technique for improving the dispersibility of silica by blending a silane coupling agent with silica is known (for example, Patent Document 1). In the examples in Japanese Unexamined Patent Application Publication No. 2000-017107A, bis(3-triethoxysilylpropyl)tetrasulfide (Si69: manufactured by Evonik Degussa Corp.) is used as a silane coupling agent (see [0027] of Japanese Unexamined Patent Application Publication No. 2000-017107A).
However, although the rubber composition for tire treads according to the above technology has better silica dispersibility than conventional compositions for tire treads, it still does not satisfy the required level.
Furthermore, another known technique for improving silica dispersibility is to blend a silane coupling agent containing a mercapto group with silica (for example, International Patent Application Publication No. WO/2006/028254).
When the present inventors studied rubber compositions for tire treads in which a silane coupling agent having a mercapto group was blended together with silica, they found that, although the required level of silica dispersibility was satisfied, Mooney viscosity was high and processability was insufficient.